Propylene resins, among olefin resins, are used in a variety of fields, such as daily goods, kitchen utensils, packaging films, consumer electronics, mechanical parts, electric components, and automobile parts, and propylene resin compositions including various types of additives are used, depending on required performances. In recent years, efforts for producing light weight, thin-walled molded articles have been made in various industrial fields, as an attempt to realize 3R (Reduce, Reuse, and Recycle) in order to achieve a recycling-oriented society. At the same time, improvements in the propylene resin compositions have been made, so that sufficient rigidity and impact resistance can be obtained even in the light weight, thin-walled molded articles.
As a polypropylene having an improved impact resistance, a polypropylene block copolymer has been produced industrially, and widely used in the above mentioned applications. This block copolymer is also referred to as an impact copolymer, or a heterophasic copolymer. Specifically, in a multistage polymerization process for producing the block copolymer, the polymerization of a homopolymer is carried out first, and then ethylene is copolymerized in a subsequent reaction tank, to produce a composition including an ethylene-propylene polymer. Since the thus produced block copolymer has a structure (sea-island structure) in which “islands” of the ethylene-propylene polymer float in the “sea” of the homopolymer, it has a better impact strength as compared to the propylene homopolymer. Note, however, that the term “block” in the “polypropylene block copolymer” does not mean it is a “block copolymer”. In other words, the polypropylene block copolymer is not composed of a homo-polypropylene chain and an ethylene-propylene copolymer chain, chemically bound to each other, but is a composition obtained by a two-stage polymerization.
For example, Patent Document 1 discloses a propylene resin composition composed of: a metallocene catalyst-based propylene block copolymer composition, in which the rubber moiety of the ethylene-propylene copolymer is constituted by two components having a low ethylene content and a high ethylene content; an elastomer; and an inorganic filler. Further, Patent Document 2 discloses a propylene block copolymer-based resin composition containing a high molecular weight propylene/ethylene copolymer rubber. Although the propylene resin composition disclosed in Patent Document 1 or Patent Document 2 has an improved impact resistance, the improvement to meet a demand for further rigidity has been insufficient.
In contrast, Patent Document 3 discloses a technique to carryout a multistage polymerization using a catalyst containing a bridged bisindenyl zirconocene capable of producing a polypropylene having a vinyl group at its terminal. In the technique disclosed in Patent Document 3, propylene is polymerized in the first stage, and the polypropylene is copolymerized with a small amount of comonomer(s) in the latter stage, so that a portion of the polypropylene having a vinyl structure at its terminal, produced in the first stage, is introduced into the main chain in the latter stage polymerization. As a result, a composition including a branched propylene copolymer composed of a grafted polypropylene can be obtained. Further, Patent Document 4 and Patent Document 5 disclose techniques to obtain a branched polymer in which the molecular weight of the side chain polypropylene is increased, by using a catalyst system carrying a bridged bisindenyl hafnocene complex. Not like the above mentioned block copolymer, the polymer produced by any of the techniques disclosed in Patent Documents 3 to 5 partially includes a branched polymer. The presence of the branched polymer improves the compatibility between the polypropylene moiety and the rubber moiety, and thus a polypropylene composition characterized by having excellent transparency and a high fusibility can be obtained. However, in the polymer produced by any of the techniques disclosed in Patent Documents 3 to 5, the melting point of the polypropylene moiety is not sufficiently high as compared to the above mentioned polypropylene block copolymer obtained using a general purpose Ziegler-Natta catalyst system, and there are limitations on the comonomer compositions of the rubber moiety and on the molecular weight. Therefore, obtaining a polypropylene resin composition which sufficiently satisfies both the rigidity and impact resistance has not yet been successful.
In view of the above, techniques have been developed to produce a block polymer (including a straight chain or a branched polymer), in which an ethylene copolymer is bound with polypropylene, and which has an excellent capability to modify a polypropylene resin.
Patent Document 6 and Patent Document 7 disclose methods in which a reactive functional group such as maleic acid, a halogen, or a leaving metal is introduced into a polyolefin, and then a coupling reaction of an ethylene/α-olefin copolymer chain with a crystalline propylene polymer chain is allowed to proceed, to produce a composition having a high target polymer content. However, in the methods disclosed therein, there is a potential risk that problems in terms of product quality could occur, such as: poor productivity due to complex reaction processes including a step of introducing the functional group into the polymer and a coupling step; coloration or foul odor due to byproducts or residual substrates produced during respective reaction steps; and contamination due to eluted components.
Patent Document 8 discloses a method for producing a straight-chain block copolymer composed of an ethylene/α-olefin copolymer chain and a crystalline polypropylene chain, using a reversible chain transfer technique. However, since this method requires a reversible chain transfer agent, it has a poor economic efficiency and its application is thereby limited. On the other hand, methods have also been disclosed in which a branched copolymer of an ethylene copolymer and polypropylene is obtained in a cost-efficient manner, utilizing a polymerization catalyst technique. For example, Patent Document 9 and Patent Document describe that a branched olefin polymer composition including a main chain soft segment constituted by an ethylene copolymer and a side chain hard segment constituted by polypropylene is useful as a polypropylene resin modifier. Patent Document 9 discloses a composition including a grafted olefin polymer having side chains composed of an ethylene polymer. Patent Document 10 discloses a composition including a grafted olefin polymer, obtained by using a specific polymerization catalyst, characterized by having excellent physical properties as a thermoplastic elastomer, such as elastic recovery, and having side chains composed of a crystalline propylene polymer.
However, although the composition disclosed in Patent Document 9 or Patent Document 10 includes a branched olefin polymer having side chains composed of a crystalline propylene polymer, it has been found that the disclosed technique has a low efficiency in producing the grafted olefin polymer, and, when the polymer is blended with a polypropylene resin to form a mixed composition, the improvement in the balance between the physical properties is insufficient. In order to obtain a branched copolymer having a high content of polypropylene side chains and an excellent modification ability, a catalyst having a good copolymerizability is required, which is capable of efficiently copolymerizing a vinyl terminated polypropylene macromonomer produced in the first stage polymerization and increasing its molecular weight in the latter stage polymerization vessel, at a high temperature (90° C. or more) at which the macromonomer is able to melt in a favorable manner.